S. aureus is a common bacteria that can cause painful skin infections or life-threatening blood or bone infections. Because it can form bacterial communities deep within bone where it can survive for long periods of time, S. aureus bone infections are extremely difficult and costly to treat. Patients with S. aureus bone infections are often treated with antibiotics and undergo surgery to remove infected tissue, but infection recurs in 40 percent of patients and amputation of infected limbs is sometimes necessary.

“The challenge with bone infections is that they tend to be incurable,” said Edward Schwarz, Ph.D., Richard and Margaret Burton Distinguished Professor in Orthopaedics and director of the CMSR. “Surgeons take extra margins around infected bone, reconstruct, and the infection comes back. They don’t understand why this infection keeps coming back and neither did we.”

To investigate, lead author, Karen de Mesy Bentley, M.S., director of the Electron Microscopy Shared Laboratory and faculty associate in the Department of Pathology and Laboratory Medicine at URMC, performed systematic transmission electron microscopy (TEM) of S. aureus infected mouse femurs and tibias. Mice were infected with S. aureus directly or via contaminated implants 14 days prior to imaging.

Bentley discovered the bacteria within the tiny channels, called canaliculi, during her many hours of TEM examination of the tissue. It appeared that S. aureus must squeeze from round to rod-shaped, which has never before been documented, to fit into canaliculi that are many times smaller in diameter than the bacteria. Bentley’s images also suggest that S. aureus can “move”, though not in the traditional sense. Since it lacks the typical appendages of motile bacteria, S. aureus must move by dividing asymmetrically and pushing daughter cells in a desired direction, such as into canaliculi.

Schwarz and Bentley originally hypothesized that the bacteria were only getting into the canaliculi when enormous pressure from the dividing bacteria forced them through. However, several images showed lone bacteria attaching to canaliculi openings in the absence of any crowding pressure. They also appeared to be dividing abnormally – aligning the plane of division near the opening rather than splitting down the middle, so their daughter cell will be extruded into the channel.

Three parallel canaliculi with varying stages of S.aureus infection. Image by Karen de Mesy Bentley, M.S.

The team further tested this in a dish and captured live video of S. aureus “crawling” through pores in a membrane that are about the same diameter as the canaliculi. This led the team to conclude that the bacteria were not getting into the channels by chance or accident, but rather by preference.

“This may explain why surgeons see reinfections, as well as several reports published in medical journals where spontaneous recurrence of infection appears 50-75 years later,” said Bentley.

“Once the bacteria get in there, they can live several hundred years after the host is dead because the bone is an inexhaustible food supply,” Schwarz added. “They’ve evolved this mechanism because it’s nirvana in there – the immune system can’t get them and they can live forever.”

The team is currently investigating human bone tissue and have preliminary evidence that S. aureus also colonizes canaliculi of patients with S. aureus bone infections. Gayle Schneider, technical associate in the Electron Microscopy Shared Laboratory, who expertly processed the mouse tissue for TEM in the current study, is now processing bone tissue from several patients with bone infections for Bentley to further investigate with TEM.